1. Field of Invention
The present invention relates to an optically addressed spatial light modulator. More specifically, the present invention relates to a spatial light modulator in which a matrix of light modulating elements is used to address a single photoaddressed liquid crystal light valve.
2. Summary of the Prior Art
Spatial light modulators consisting of a matrix of electrically addressed liquid crystal elements can be used to modulate high intensity light and with suitable optics project an image onto a screen where it may be viewed. The quality of the image is influenced by the brightness and contrast of the projected image.
The matrix of liquid crystal elements consists of an active area (region where the liquid crystal can modulate the input light), and an inactive area (region composed of electrode data lines, busbars, electrical devices and the isolation separating adjacent picture elements). The ratio of active to inactive area is defined as the aperture ratio.
As the number of pixels per unit area is increased to achieve higher resolution, the number of components in the inactive area is also increased. The process of lithography used to define the components making up the inactive area is constrained as to minimum feature size. Consequently, the inactive area will increase and the aperture ratio, picture brightness, and contrast will accordingly be reduced.
Prior art addresses the brightness problem by separating the light used to create projected images from the light transmitted through the matrix liquid crystal element. This is accomplished by modulating a low intensity light source by a matrix addressed liquid crystal light valve. The light that is thus modulated falls onto a photoaddressed liquid crystal light valve. The image of the addressing matrix is thus reproduced at the photoconductive surface. This device has been described in copending U.S. Pat. Application Ser. No. 436,447 (hereafter referred to as Haven et al.), filed on November 14, 1989, and assigned to the same assignee as the present application.
Another problem with the prior art is that the optical properties of a photoaddressed light valve are spatially modulated according to the intensity of the addressing light at each picture element. Therefore, any light falling outside defined pixel boundaries will degrade system contrast. Prior art requires expensive collimating optics together with fiber-optic substrates to provide good contrast.
An additional problem with prior art is the effect of diffracted energy outside the pixel boundaries as the pixel size in the matrix modulator becomes small (higher resolution). Since the active area in the matrix modulator defines the pixel boundaries, contrast will be degraded as resolution increases.
Additional prior art, represented by U.S. Pat. No. 4,836,652 by Oishi et al. is directed toward a similar application, that of electrographic imaging using light projected through matrix addressed liquid crystal shutters to address a photosensitive medium. In this case, light is focused by an array of lenses and directed through the matrix modulator element onto a photosensitive medium, with unwanted or diffuse light blocked by a photomask. The disadvantages of the approach outlined by Oishi et al. are (1) the glass substrates must be thin, and consequently fragile, so that light directed from the matrix element does not diffuse into adjacent elements and degrade contrast, (2) the photosensitive medium must be in close proximity to the photomask to minimize diffraction effects from the photomask and consequent loss of resolution and contrast, (3) the size of the light opaque region of the photomasks aperture must be large to satisfy the geometry constraints on the lens design. Therefore, no aperture ratio improvement can be realized.
Earlier prior art in this area relies on the use of lens array (Selfoc) disposed between the matrix modulator and the photosensitive medium, to image the matrix onto the medium. This limits the projected geometry to that of the matrix aperture itself. Therefore no aperture ratio improvement can be realized.